Embodiments of the invention relate generally to motor drive systems for operating a three-phase electric motor and, more particularly, to an apparatus and method for operating a three-phase motor from a single phase power source.
One type of system commonly used in industry that performs power conversion is an adjustable or variable speed drive (ASD) circuit, which is an industrial control device that provides for variable frequency, variable voltage operation of a driven system, such as a three-phase AC induction motor. ASDs have an AC to DC rectifier unit with a large DC capacitor to smooth the voltage ripple. In all ASDs, the DC bus voltage is inverted to three-phase variable voltage, variable frequency output to control the speed and torque of three-phase AC motors. For many loads, it is customary and often required to power up the AC to DC rectifier section from a three-phase AC source. However, the input AC to DC rectifier can be powered up from a single-phase AC source, especially in locations where three-phase AC power is unavailable due to logistics and other reasons. In such cases, some utilities allow three-phase ASDs to be powered from a single-phase AC source provided the peak current flowing into the AC to DC rectifier system is within the rating of the single-phase AC source. Many ASD manufacturers impose restrictions on the rating of the ASD when they are subject to a single phase AC source.
There exist important concerns while operating a three-phase ASD from a single-phase AC source—with such concerns tied to high peak current and zero current conditions associated with a single phase AC power, as can be seen in the single phase AC waveform shown in FIG. 1 by high peak current 10 and zero current 12. With respect to the input AC current provided from the single phase AC source to the ASD, it is recognized that the input diodes of the ASD have to handle the higher demand current for a given load (with the RMS value of the input AC current being significantly higher than that when a three-phase supply is used for a given load), and thus de-rating of the inverter is often undertaken to address this concern—with the de-rating being ˜50% in some cases. The higher input current affects the input AC power terminal blocks and, while in many cases the rectifier diodes of the ASD may be able to handle the higher values of RMS current, the terminal blocks and the rectifier diodes of the ASD may not be rated to handle the peak current on a continuous basis. An additional concern is that input harmonic distortion is high when single-phase input is used as an AC source for a three-phase inverter. Poor harmonics are associated with lower input power factor, which affects the efficiency of power conversion. Single-phase AC supply results in higher ripple voltage across the DC bus. Higher ripple voltage translates to higher ripple current through the DC bus capacitors and more heating of these capacitors. The inverter is typically de-rated to handle the higher ripple current. The current drawn from the single-phase AC source feeding a three-phase ASD is discontinuous. When the pulsed current flows from the AC source, it creates voltage drop that mimics the pulsed current waveform to some extent. The resulting voltage drop can affect other loads connected to the same AC source.
There are many known techniques that are employed to improve the current waveform and reduce the overall current harmonics—including both passive and active techniques. One known passive approach creates a resonant circuit across the DC bus, with energy being stored in the resonant components and released naturally at the appropriate time to support the sagging DC bus voltage and thereby reduce the ripple across the bulk capacitors of the DC bus. However, while the resonant circuit provides benefits of extending the diode conduction period during the charging cycle to reduce the input harmonics and improve the input power factor and reducing the DC bus capacitor ripple, it is recognized that the passive resonant circuit has disadvantages as well. These disadvantages include that the resonant components are bulky and expensive, the peak diode current is reduced but the improvement is not conspicuous, and the average DC bus voltage is still low and the ASDs need to be de-rated, though the level of de-rating is smaller than that without the DC bus resonant circuit.
One active solution commonly used in single-phase AC to DC power supplies uses a boost converter that boosts the input voltage to a desired DC bus voltage level under all load conditions. Use of the boost converter beneficially reduces the overall DC bus voltage ripple and makes the input current continuous, which reduces the input current harmonic distortion (thereby eliminating the peak current stress in the input diodes) and resulting in lower thermal loss in the AC system. However, it is recognized that the boost converter (and operation thereof) has disadvantages associated therewith of being expensive due to the boost switch having to be rated to carry peak of input current and to handle the boosted voltage (i.e., a large stress across the switch of the boost converter) and of requiring an input EMI filter to limit switching noise observed in the input AC voltage waveform from propagating into the AC source.
One particular active solution used to improve the current waveform and reduce the overall current harmonics is disclosed in U.S. application Ser. No. 14/672,967. FIG. 2 illustrates a single phase front end circuit 14 for use with a ASD 16, where the front end circuit 14 includes a bidirectional switch 18 positioned between an input to the rectifier 20 and a midpoint 22 (formed by a pair of capacitors) positioned across the rails of the DC bus 24, so as to form a single-phase partial boost converter. The bidirectional switch 18 is controlled to inject current into the midpoint of the DC bus 24 even during the time when the input AC voltage is lower than the DC bus voltage, with such current being typically only about 50% of the rated current of the AC to DC rectifier. The current flow is limited by an external inductor 26, which behaves like a boost inductor boosting the main DC bus voltage when the switch is forced to turn of at sometime near to the peak of input AC voltage.
However, while the front end circuit 14 operates to successfully reduce the ripple across the DC bus capacitor 28 and reduce the peak current flowing through the diodes of rectifier 20, the front end circuit 14 does not provide an optimum solution for conditioning the single phase current waveform and reducing the overall current harmonics, including such concerns tied to a high peak current and zero current conditions. As seen in FIG. 3, which illustrates a current waveform 30 output from the front end rectifier circuit 14 resulting from operation thereof (i.e., from operation of bidirectional switch 18), the current provided by the front end rectifier circuit 14 is non-sinusoidal—such that a large amount of total harmonic distortion is still present in the waveform. Additionally, periods/intervals are present where zero current (indicated at 32) is output from the front end circuit 14, which is problematic for operation ASD 16 in an optimum fashion.
It would therefore be desirable to have a system and method for operating a three-phase motor from a single phase power source that overcomes the aforementioned drawbacks. It would also be desirable for such a system and method to utilize a standard ASD with an add-on kit, to provide a non-intrusive solution that minimizes the cost, weight and size of the system in single phase applications, minimizing the need of large derating. It would still further be desirable for such a system and method to provide an input current harmonics reduction that makes it possible to meet industry regulatory power quality mandatory requirements.